Antennas having a radiation pattern in the form of several fixed diverging beams are often used in tracking a moving source of radiant energy, as in telemetry applications, wherein it is desired to communicate with a moving vehicle carrying the source as well as to determine the direction of the vehicle and source relative to the antenna. Electrical circuitry of the prior art coupled to such an antenna senses the relative magnitudes of signal strength in a plurality of the beams to sense the direction of the source.
A problem arises in that, while accurate sensing of the source direction is obtainable when the source is oriented along the axis between adjacent beams, the capability for sensing the source direction is continuously diminished as the source moves off axis towards one of the beams of the aforementioned pair of beams. This is in accordance with the well-known decreasing slope of the angle error graph for sources of radiation oriented away from the null between two receiving beams in a monopulse radar.
A further problem arises in the communication along the telemetry link to the vehicle, particularly with communication employing a phase modulation of the carrier of the radiation transmitted from the vehicle to the antenna. This problem arises because the signal processing of electrical circuitry which senses the relative magnitudes of the signals of the beams may introduce phase modulations in addition to those communicated from the vehicle, these additional phase modulations degrading the quality of the received signal.